The hippocampal region plays a critical role in declarative memory. In addition, there have been significant advances in characterizing several defining features of declarative memory, and in identifying the cortical and hippocampal structures essential to this form of memory. Yet, we have little insight into the nature of the neural coding that underlies declarative representations by the hippocampus or cortex, or into the nature of the essential cortical-hippocampal interactions. The long- term objective of this project is to clarify the hippocampal "memory code" by analyzing firing patterns of single neurons and neuronal ensembles of cortical and hippocampal areas in rats performing different memory tasks, and to identify aspects of cortical memory coding dependent on components of the hippocampal system. Previous anatomical, neuropsychological, and neurophysiological studies have identified the set of cortical structures and pathways by which these structures interact with the hippocampal region, and have provided a preliminary identification of which components of this systems are important to particular features of hippocampal-dependent memory that can be examined in animals: (1) The cortex and parahippocampal region (perirhinal and entorhinal cortex) mediate the sustained retention of memories for single brief experiences. We will examine the nature of cortical representations in this process, and their dependence on the parahippocampal region in sustaining memory representations. (2) Structures of the hippocampal region mediate the organization and the consolidation of cortical memory representations. We will characterize the nature of cortical representations of stimulus- stimulus associations and determine the extent to which the organization of cortical representations depends on hippocampal function. (3) The hippocampus itself may play an especially important role in learning stimulus configurations and episodic information. We will examine the development and scope of the encoding of stimulus configurations in the hippocampus and determine if hippocampal codings are organized to represent episodic memories. The proposed studies will exploit a well established olfactory- hippocampal model system that exploits the superb capacities of rats for odor-guided learning and memory, and is based on well known anatomical and physiological data about olfactory and hippocampal structures and their interactions. Furthermore, the proposed studies will employ recently developed methods for the collection and analysis of neural ensemble recordings in the cortex and hippocampal region. The findings from this work will improve our understanding of 'what the hippocampus does' and what it does within the system of cortical structures with which it interacts in the service of declarative memory.